The possibility of a non-invasive diagnosis of blood glucose levels became a subject of researchers' interest over 30 years ago. Since then, there have been more than a dozen different methods of the measure thereof, based on fundamentally different effects. However, optical methods for determining concentration of glucose in blood were and remain the most attractive ones. The main advantage of such methods is primarily human safety. Numerous experiments confirm the possibility of development of a non-invasive blood glucose meter (NG) based on optical methods (Bazaev N. A., Masloboev J. P., Selishchev S. V. Optical Methods of Noninvasive Determination of Blood Glucose Levels. Medical Facilities 2011, No 6 (270) S.29-33). The reasons that prior attempts at developing NG did not succeed lie in the peculiarities of the physiology of each individual, the difficulties of interpreting the results obtained, the need for selection of optimal instrument calibration and so on.
During development of NG in the 1980s and 1990s there were high hopes for spectrophotometric methods. The visible part of the spectrum is not suitable for these measurements since glucose is substantially transparent, that is, glucose has weak light absorption. Therefore, efforts have been directed at creating a spectrophotometric NG in the infrared spectral region. The main obstacle in this field is the presence of a large amount of water in biological tissue, which strongly absorbs infrared light. Nevertheless, there are three “transparency windows” in the following wavelength ranges: 1) below 1.35 μm; 2) 1.55-1.85 μm; and 3) 2.1-2.3 μm. In the second and third ranges, there are absorption peaks specific to glucose, which were used to determine the concentration of glucose in the blood. Multi-wavelength spectrophotometry techniques were used in such determinations. It was possible to obtain and measure small concentrations of glucose with the help of calibrated solutions of glucose and background materials. However, when used for actual biological objects—for example human fingers—difficulties arose, and so the developers failed to bring the described methodology to the prototype level.
For this reason, over the last decade, many attempts have been made to measure glucose concentrations in human blood by polarimetry. Polarimetry is used for the quantitative analysis of solutions with optically active substances, such as glucose. Such materials rotate the plane of polarization of a polarized beam transmitted therethrough by the angle of α=αsp*l*c, where α, αsp are the angle of the polarization plane rotation and its specific value; c is the concentration of glucose, and l is the optical path length. The specific value of the angle of the polarization plane rotation for glucose is +56.2°[1/(g/dL)*dm].
Given that the average length of the optical path of blood vessels in the human finger is on the order of about 1 mm, the change in glucose concentration at 1 mg/dL will produce rotation of the polarization plane of only about 0.000562°, that is, a little over 2 seconds of arc. Determination of such angles by measuring the change in the intensity of the light beam when the analyzer is rotated at such angles is extremely difficult, and remains unsolved in the art. This is due to three factors. First, the intensity change due to rotation of the polarization plane for so little angle is extremely low. Second, the intensity of the light beam passing through a “polarizer-finger-analyzer (second polarizer)” arrangement is affected not only by the polarization plane rotation in a finger, which is related to the concentration of glucose in the blood, but also by numerous physical and biological processes occurring in the finger, which are impossible to take into account. Third, it is very difficult to accurately measure the rotation of a mechanical part by an angle of a few arc seconds. These three factors, in fact, are the obstacles that still have not been overcome.